Ilford FP4 Plus 125

Datasheet-derived original. Primary source: Ilford FP4 Plus technical information, Nov 2018. Archived at X-T5/_reference-sources/datasheets/ilford/fp4-plus.pdf, 6 pages, read in full. Settings in the notes; video in the notes.

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What the datasheet says

Property	Ilford's wording
Identity	"an exceptionally fine grain, medium speed black and white film", ISO 125/22°
Intended use	"ideal for high quality indoor and outdoor photography, particularly when giant enlargements are to be made"
Also	"suited to copying and internegative work, and has many applications in scientific, technical and industrial photography"
Latitude	"robust and will give usable results even if it is overexposed by as much as six stops, or underexposed by two stops"
EI range	Best at 125; good image quality from EI 50/18 to EI 200/24
Beyond that	EI ≤25 and EI ≥400 appear only under "accidental exposure only" (PERCEPTOL 8½ min / MICROPHEN 16 min)
Reciprocity	"For exposures between ½ and 1/10,000 second, no adjustments are needed"; beyond that Ta = Tm^1.26
Robustness	"very robust in processing and will tolerate less than ideal processing conditions"
Speed measured in	ILFORD ID-11 at 20 °C, intermittent agitation, spiral tank
Characteristic curve	ILFOTEC HC (1+31), 8 min at 20 °C
Developer roles	PERCEPTOL = finest grain · ID—(1—) = maximum sharpness · MICROPHEN = maximum film speed · ILFOTEC DD-X / ID-11 stock = best overall
Formats	35 mm, 120, and sheet film on 0.180 mm polyester

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Finding 1: this is the widest documented latitude in the bank, and it is asymmetric

"overexposed by as much as six stops, or underexposed by two stops."

Eight stops of usable range, weighted three-to-one toward overexposure. For comparison, against the other explicitly-quantified latitude claims in this collection:

Film	Published claim	Stops	Shape
FP4 Plus 125	6 over / 2 under, usable results	8	Strongly asymmetric
XP2 Super 400	EI 50–800 on one roll, standard processing	4	Asymmetric (3 over, 1 under of box)
Delta 400	EI 200–3200 development range	4	Push-weighted
SFX 200	"wide exposure latitude" (unquantified)	—	—
Delta 100	EI 50–200	2	Symmetric

Note the two claims are different in kind and should not be conflated. XP2's is a rating claim. Shoot the whole roll at any EI in that band and process normally. FP4's is a frame-level robustness claim, an individual mis-exposed frame still prints. FP4's is the larger number and the more forgiving property in the field.

The asymmetry is the actionable part, and it is what separates this recipe from the attributed one. Ilford is not saying "FP4 is forgiving." It is saying FP4 forgives you in one specific direction, and the direction is more light.

How that becomes settings

Dynamic Range is a highlight-protection control. It works by underexposing the sensor and lifting the shadows back, buying headroom at the top. That maps exactly onto a six-stop overexposure tolerance, so —, the maximum the X-T5 offers. Is the direct translation, with Highlight —softening the shoulder on top of it.

And the shadow end gets nothing, because the sheet gives it nothing. Two stops of underexposure tolerance is unremarkable. Most conventional B&W films manage that, and there is no shadow-detail, open-shadow or long-toe language anywhere in the six pages.

The shadow decision

This is the recipe's most contestable value and it deserves the argument in full, because it departs from both the attributed reference recipe (Shadow —) and from FP4's widely-repeated reputation.

The reputation: FP4 "rarely goes fully black" and has "excellent shadow detail" That phrasing appears in the reference recipe's own description and across the photography community. It would argue for a lifted, negative Shadow value.

What the datasheet actually supports: nothing of the kind, explicitly. What it says is that the film tolerates two stops under, which is normal. Against six over. If anything, the document points the opposite way: this is a film whose margin lives at the top of the curve.

So the recipe runs —. Neutral. Not lifted, not deepened.

I could be wrong about this, and the test that would prove it is specified in the notes: shoot a deliberate two-stop underexposure. If the shadows hold gracefully, if the film's toe really is long, then the reputation is describing something real that Ilford simply did not write down, — is too conservative, and the reference recipe's −1.5 was right on evidence the sheet doesn't carry. That is a falsifiable claim and it should be tested rather than argued.

Until then, the discipline this bank applies elsewhere holds: the datasheet outranks the folklore, and where the datasheet is silent the default stands. It is the same standard that gave Delta 100 — despite marketing prose that sounded like Pan F's.

The Monochrome-plus-lifted-shadows reading is preserved as a named variant, credited to the reference recipe, so nothing is lost if the test goes the other way.

The — decision, and its cost

— pins ISO to a minimum of 500 on the X-T5. On a film rated ISO 125, that is a two-stop mismatch, and it is worth naming rather than glossing.

Precedent in the bank runs both ways: Delta 100, Acros 100 and Pan F Plus 50 all use — (ISO floor 250) at similar or slower speeds. Following that convention would put FP4 at —.

The reason to break it: the convention is a proxy for "slow film, narrow latitude," and FP4 is the counterexample, a slow film with the widest documented latitude in the collection. Setting it to — would model the speed while ignoring the specification, which inverts the bank's own priority. The same reasoning put XP2 Super on —.

Why the mismatch matters less than it looks: the ISO number in a digital recipe does not simulate film speed. Grain does, and grain is set independently at —/—. ISO 500 on an X-T5 is essentially noise-free, so the floor costs nothing visible. What it costs is shutter/aperture flexibility in bright light, which is a real but manageable constraint. Anyone shooting wide open in full sun should use the — variant and accept two stops less highlight headroom.

Finding 2: the best long-exposure characteristic in the bank

Ilford publishes reciprocity-failure correction as an explicit formula, which most manufacturers do not:

Ta = Tm^1.26, with "no adjustments needed" from ½ second to 1/10,000 second.

Two comparable data points exist in this collection, both from Ilford sheets read in this repo:

Film	Exponent	No-correction threshold
FP4 Plus 125	1.26	½ second
XP2 Super 400	1.31	1 second
Kodachrome 64	(no formula)	1/10 s needs +⅓ stop; 1 s and beyond "not recommended"

A lower exponent means less compensation required, compounding as exposures lengthen:

Metered	FP4 (^1.26)	XP2 (^1.31)	FP4 saves
30 s	1 min 13 s	1 min 26 s	13 s
60 s	2 min 54 s	3 min 34 s	40 s
2 min	6 min 56 s	8 min 49 s	1 min 53 s
5 min	22 min	29 min	7 min 16 s

This does not change a single digital setting, the X-T5 has no reciprocity failure, and it is recorded honestly as such. It matters for two reasons. First, it tells you what the film was for: architecture, interiors, night, ND-filter water work, the technical and scientific applications the sheet names. Second, if you ever shoot a comparison frame on real FP4, this is the constraint the film is working under and the recipe is not.

The XP2 comparison is the more interesting one, and it inverts an assumption: the chromogenic film with the famously huge rating range has the worse long-exposure behaviour of the two. Latitude and reciprocity are independent properties, and this bank now has the numbers to say so.

Base and Clarity: same standard, applied again

Acros over Monochrome. The reference recipe's Monochrome choice is defensible and I said so in its analysis, a flatter base suits a gentle, unhurried film. Reading the datasheet changes the reasoning: FP4 is sold on "high print quality", "giant enlargements", and copying, internegative, scientific, technical and industrial work. That is a precision instrument, not a gentle one, and Acros. Fujifilm's premium B&W profile, with richer tonality and better detail than plain Monochrome. Is the correct base for it. The Monochrome reading survives as a variant.

—. The sheet contains no micro-contrast, edge-contrast or acutance language across six pages. The bank's standard is explicit and has now been applied four times, Pan F Plus 50 earns Clarity —ecause its sheet says "edge contrast"; Delta 100, XP2 Super and now FP4 stay at 0 because theirs do not. The reference recipe's +2 is unsupported by the document.

Sharpness —, not —. The resolution claims are real. Giant enlargements, copying, and ID-11 (1+3) listed for "maximum sharpness" in the developer table, so it goes positive. But Pan F's +2 tier is tied to that edge-contrast language, and consistency means FP4 lands one notch below.

What this derivation corrects

Three corrections from one document that had been archived in this repo, unread, since 2026-07-15:

Setting	Reference recipe	This original	Basis
Grain	Weak, Large	Weak, Small	"exceptionally fine grain"; PERCEPTOL = finest grain. Weak/Large is a combination no datasheet describes
Clarity	+2	0	No edge-contrast language anywhere in the sheet
Shadow	−1.5	0	Latitude is documented as asymmetric toward overexposure; no shadow claim is made
DR	—	—	The six-stop overexposure specification
Base	Monochrome	Acros	Giant-enlargement, copying and technical-photography claims
Sharpness	0	+1	Three separate resolution claims
EC	−1 to −1/3	0 to +2/3	A sign reversal — see below

The exposure-compensation reversal is the largest disagreement, and it is worth being explicit about. The reference recipe runs negative EC on the reasoning that "slight underexposure protects highlights and builds contrast" and matches "the sheet's tight EI 50–200 latitude."

Reading the sheet, that premise does not hold. The EI 50–200 band is the quality range, not the latitude, the latitude is separately specified as six stops over and two under. Underexposing a film whose documented tolerance is three times better in the other direction is moving toward its weak end, not its strong one. The 2026-07-15 audit accepted the negative-EC convention as ✓; that acceptance was based on the EI range and should be revised now the actual latitude specification has been read.

Where it sits in the B&W set

Speed	Grain	Latitude claim	Character
50	Weak/Small	—	Slowest, sharpest, etched (Clarity —)
100	Weak/Small	EI 50–200	T-grain precision, quiet
100	Weak/Small	—	Clean, smooth, wide tonality — 4 scans
125	Weak/Small	6 over / 2 under	The forgiving one
200	Weak/Small	wide (unquantified)	Extended red to 740 nm
400	Weak/Small	EI 50–800 one roll	Chromogenic, the clean 400
400	Strong/Small	—	Forgiving reportage — best skin evidence
400	Strong/Small	EI 200–3200, Gbar 0.62	Modern T-grain precision
400	Strong/Large	—	Gritty, punchy, Clarity —
1000 native	Strong/Large	EI-3200-by-design	Heaviest grain, pushed

Ten B&W originals, and the Ilford conventional-grain line is now complete: Pan F 50 → FP4 125 → HP5 400. That sits alongside the tabular Delta line (100/400/3200) and the chromogenic XP2, three emulsion technologies from one manufacturer, all datasheet-derived, which is a genuinely unusual thing for a recipe bank to hold.

Validation status

Datasheet-validated, no scan. Every setting traces to the archived sheet, and the four places this recipe disagrees with the attributed reference are argued against specific document language above rather than asserted.

Highest-value evidence to add, in order:

1. A frame with an extreme brightness range, the six-stop claim is this film's identity and one frame settles it.
2. A deliberate two-stop underexposure. Tests the — decision directly, and would vindicate the reference recipe if the shadows hold better than the sheet implies.
3. A large-detail subject at 100%. Brickwork, weave, foliage. Testing whether Sharpness —/ — delivers the enlargement claim.

Drop any of these into references/.

Derived: 2026-07-25